Direct current (dc) motors produce mechanical torque from a direct current or non-varying electrical current source. It is advantageous to employ a dc motor in many applications since it is not necessary to go through the step of converting the output of available portable dc power sources, such as electrochemical batteries, fuel cells or solar cells, to provide an input suitable for running a more highly efficient ac motor.
Various electro-mechanical arrangements have been proposed for switching constant direct current to produce mechanical torque. For example, in the case of automotive starter motors, an electro-mechanical arrangement is employed which switches the direction of current through a "rotor" coil on a moving rotor as it is rotated through a "stator" magnetic field. The rotating coil produces a rotor-position-varying magnetic field, i.e., flux vector, which interacts with the constant stator magnetic field to produce torque. The arrangement for switching constant dc to produce a rotational-varying flux vector is commonly termed "commutation" of a dc motor. This is typically accomplished by a pair of switches being in contact with the rotor to switch polarity of the rotor magnetic field to insure that the rotor is continuously attracted in the same direction. This can be a source of trouble in purely electro-mechanical motors, however, since often stationary mechanical parts are abraded by moving parts, and because electrical current flowing across the junction frequently produces accumulations of dirt, oxidation, and electrical noise due to arcing.
A more satisfactory system for commutation was developed when a permanent magnet was placed on the rotor of the dc motor and, by various electronic means, a time or rotational varying magnetic field was created in the stator coils. Thus, small permanent magnet rotor motors have been available which are more efficient than the prior electro-mechanical versions due to lower magnetic losses. Such permanent magnet dc motors have been referred to as "stepper" motors because the field coils are stepped through a sequence of discrete flux steps to cause the rotor to shift to discrete shaft positions. By employing a network of appropriate electronic "switches", a sequence of "steps" are produced to move the shaft of the motor. Although such a "stepper" approach is adequate for discrete precision positioning, it obviously is unacceptable where smooth torque generation is desired, since the discrete steps which are created by such a system will produce "cogging", or torque variation with respect to time, unless a critical "tuning" of the mechanical system is performed for each step motion.
Sequential energization of the driving coils of a brushless dc motor in response to the angular position of the rotor has been proposed in U.S. Pat. No. 3,900,780 issued to Tanikoshi on Aug. 19, 1975, wherein a light-shielding disk mounted coaxially with the rotary magnet is employed together with light sensors to create energization signals; and also in U.S. Pat. No. 4,525,657 issued to Nakase et. al. on Jun. 25, 1985, wherein a rotary encoder disk is fixed to the rotor magnet shaft and is provided with a series of slits to create a series of pulsed signals which are used to energize the stator coils. The use of optically encoded wheels which are secured to the rotor of a brushless dc motor in order to sense the rotor position, and for utilizing the signal obtained from such sensors to control the energization of specific windings of the stator such that a rotating electromagnetic field is provided in synchronization with the rotation of the magnetic poles of the rotor, is further suggested in U.S. Pat. No. 4,353,016, issued on Oct. 5, 1982 to Born. In the Born device, the code wheel is a multi-sector disk which has as many optically detectable transitions as there are magnetic poles. Born processes the resulting square waves from the sensors through a filter network to provide sinusoidal motor drive signals which continuously modulate motor winding currents to provide linear speed control.
Such prior art devices all employ involved circuitry for converting square wave pulsed signals into appropriate signals which are then fed to the stator windings to insure movement of the rotor. There is no suggestion of employing an optically encoded disk which creates a signal which is itself sinusoidal, and which only requires linear amplification in order to provide power to the stator coils in a rotor position-related sequence.
A more recent suggestion for improvement of brushless dc motor technology involves feeding the position of the rotor shaft to a microcomputer and then energizing the stator coils in a digitally synthesized sinusoidal manner to produce smooth torque. However, such a system is subject to problems such as high electro-magnetic fields which may be caused by lightning strikes, and also errors which may be introduced by program designers.
Still other art relating to the present invention is described and illustrated in U.S. Pat. No. 3,883,785 issued to Fulcher et al. on May 13, 1975; U.S. Pat. No. 4,463,291 issued to Usry on Jul. 31, 1984; U.S. Pat. No. 4,717,864 issued to Fultz on Jan. 5, 1988; and U.S. Pat. No. 4,899,093 issued to Gleim on Feb. 6, 1990.